1. Field of the Invention
The present invention relates to a backlight module, and more particularly to a backlight module comprising a light mixing space positioned between a light guide plate and a diffuser sheet.
2. Description of the Prior Art
As quality demands increase rapidly for display devices, LCDs are becoming a major type of display devices today, finding wide application to notebooks, personal digital assistants, digital cameras and other portable electronic products. Because the LCD is a passive illuminating display device, a backlight module is often necessary to provide a light source for it.
A conventional backlight module of the LCD uses a cold cathode fluorescent lamp (CCFL) as the light source. Taking a direct-lighting type backlight module as an example, a plurality of CCFLs are positioned collaterally on the bottom of the backlight module to provide a plurality of line-like sources. A diffuser sheet, or a plurality of optical films, is placed on the CCFL to provide planar light with uniform brightness. The backlight module with the CCFL as the light source is a well developed technology, thus the diffuser and the optical film are designed well enough to provide an efficient light source. However, the large size of the CCFL and the high illuminating heat become restrictions of LCDs, so the CCFL no longer satisfy the customers' demands. In addition, the CCFL contains mercury, which causes environment pollution. Therefore, the light emitting diode (LED), which has advantages of high color saturation, quick reaction rate, and small size, is used widely in the backlight module as a light source. Research groups are still working on a backlight module incorporating LEDs for large-sized LCDs to replace the conventional cold cathode fluorescent lamp. However, because the color saturation of white LEDs still needs to be improved, red LEDs, green LEDs, and blue LEDs are used simultaneously as the light source in the backlight module to replace the white LEDs. In this situation, a design with good light mixing is important for making the red, green, and blue light mix sufficiently. Therefore, the backlight module incorporating LEDs has a more complicate structure and higher price than the backlight module incorporating the CCFL.
FIG. 1 shows a side view of the conventional backlight module incorporating LEDs. The conventional backlight module 10 is a direct-lighting type backlight module applied to large size LCDs, such as flat panel TVs. The backlight module comprises a frame 12, a diffuser sheet 20 positioned on the frame 12, and a plurality of top-emitting LED sources 14 positioned in the frame 12 and soldered on a printed circuit board (PCB) 18. The LED source 14 comprises a red LED, a blue LED, and a green LED positioned uniformly in the frame 12. After mixing the red, blue, and green light properly, a white light source can be provided to LCDs. In addition, the conventional backlight module further comprises a reflective sheet 16 or reflective material positioned on the PCB 18. Usually, the thickness of backlight module of a flat panel TV is approximately 2 cm. Therefore, in order to achieve uniform brightness, higher numbers of LED sources 14 with smaller current, such as 20 mA, are positioned in the backlight module 10. In this design, the light emitted by the LED sources 14 illuminates the luminous face of the backlight module 10 directly, so the light utility efficiency is higher and the heat dissipation is better, because the LED sources 14 are positioned uniformly. However, the increased number of LED sources 14 increases the cost of the backlight module 10. In addition, according to the structural design of the backlight module 10, the light cannot mix well. Furthermore, only a specific wavelength range or power of the red LED, blue LED, and green LED can be used in the LED source 14.
Moreover, the conventional backlight module can also include a lower number of high-power LEDs, such as 350 mA LEDs, combined with special design prisms positioned on the surface of the LED to provide the light source. However, this design increases the thickness of the backlight module, and fans, heat pipes, or other heat dissipation devices are needed to help heat dissipation, so the cost is increased. In addition, special design prisms are usually patented, so the product cost of the backlight module is also increased.
As mentioned above, designing an LED backlight module with low cost and good light mixing function is an important task for the backlight module industry.